Exhaust gas recirculation (EGR) circuits are known in the art as a method to modulate a combustion reaction within an internal combustion engine. Such EGR circuits remove a portion of exhaust gas flow from the exhaust system. Exhaust systems transport combustion by-products in the form of exhaust gas flow from the engine through various treatment devices and out of the vehicle through a tailpipe. EGR circuits channel a portion of exhaust gas flow back to an input flow to reenter the combustion chambers within cylinders of the engine. In such an application, the exhaust gas flow, when mixed with the fuel air charge within the combustion chamber, acts as an inert gas, changing the properties of combustion within the chamber. The effects associated with the use of EGR, for example, the reduction of NOx emissions, are known in the art. EGR circuits are known for use in many different engine types and configurations, for instance in both diesel and gasoline engines.
Combustion, the process by which a fuel air charge is ignited and utilized to create work in a combustion chamber, is highly dependent upon the conditions existing within the combustion chamber. Variations in properties such as temperature within the combustion chamber can cause adverse effects upon the resulting combustion. The temperature of the EGR flow channeled into the combustion chamber has effects upon the overall temperature within the combustion chamber. As a result of the need to control these temperatures, methods are known to modulate the temperature of EGR flow within the EGR circuit through the use of an EGR cooler comprising a heat exchange device.
Heat exchange devices can take many forms. One known heat exchange device is a gas to liquid type heat exchanger, wherein a gas flow is passed through a plurality of gas flow passages defined by walls within the heat exchanger, and wherein a liquid flow is passed through a plurality of liquid flow passages defined by walls within the heat exchanger. One known liquid used to cool the EGR flow within the heat exchanger is engine coolant, frequently in communication with the engine cooling system; however, it will be appreciated many different liquids, either as part of an existing liquid circuit in the vehicle or as a dedicated circuit for use by the EGR cooler, can be used for the heat exchanger. Another known heat exchange device is a gas to gas type heat exchanger, wherein a first gas flow is passed through a plurality of gas flow passages defined by walls within the heat exchanger, and wherein a second gas flow is passed through a second plurality of gas flow passages defined by walls within the heat exchanger. An air flow channeled from outside the vehicle through the heat exchanger is frequently used as a cooling gas flow, although it will be appreciated many different gases, either as part of an existing liquid circuit in the vehicle or as a dedicated circuit for use by the EGR cooler, can be used for the heat exchanger. Additionally, multiple stage EGR coolers are known, wherein the EGR flow is passed through a plurality of heat exchangers in series, the first heat exchanger cooling the EGR flow to some intermediate temperature and the second heat exchanger cooling the EGR flow to some cooler temperature. Alternatively or additionally, heat exchangers can be utilized in parallel, with the EGR flow being directed between one path or the other, with each path containing a single heat exchanger or multiple heat exchangers in series. In such multiple stage EGR coolers, different types of heat exchangers or different cooling mediums can be utilized. Also, in some circumstances, the EGR cooler can actually be used to impart heat to the EGR flow from another medium to the EGR flow, for instance, in an engine warm-up condition. The walls within the heat exchanger defining the gas flow passages for the EGR flow are frequently the same piece of material as the walls within the heat exchanger defining the flow passages for the second flow, where the flows are in contact with opposite sides of the piece of material. By utilizing such designs, flows of two distinct materials flowing on either side of the walls can cause heat to transfer from a flow with a higher temperature to a flow with a lower temperature through the separating piece of material. Design of heat exchangers, including design of walls within the heat exchanger, choice of materials or coatings for the walls in the heat exchanger, use and design of fins within the passages to increase surface area within the heat exchanger, and other considerations are known in the art and will not be discussed herein. Additionally, heat exchangers are known in a wide variety of configurations, for example including parallel-flow, cross-flow, and counter-flow, and many interior designs of heat exchanger are known, for example wherein the liquid flow can be passed through the heat exchanger in a single pass or partitions may be used to make the liquid travel through the heat exchanger in multiple passes. Although exemplary forms of heat exchangers are described and illustrated herein, heat exchangers can take many forms and alternative embodiments, and the methods described herein are not intended to be limited to the specific embodiments described. For the purposes of this disclosure, in order to affect effective heat transfer within the heat exchanger, heat exchanger design for use in an EGR cooler requires a gas flow to go through flow passages designed to maximize the surface area through which heat can transfer between the different medium flows.
EGR flows, the exhaust gas flow tapped from the exhaust system for the purposes of controlling combustion within the combustion chamber as described above, contain by-products of combustion. Particulate matter (PM) and other combustion by-products travel through the exhaust system with the exhaust gas flow. The EGR circuit, by tapping into the exhaust system, is exposed to these by-products. As described above, heat exchanger design includes the creation of narrow and subdivided passages in order to maximize heat transfer from the hot gas to the cooling liquid. However, narrow passages with large surface areas can act as filters to the combustion by-products, collecting particulate deposits on the surfaces within the passages. Such deposits within the heat exchanger can have a number of adverse effects upon the heat exchanger, including but not limited to corrosion, increased flow resistance, flow blockage, reduction of heat transfer capacity, and NVH.
A method to reduce the build-up of deposits within an EGR cooler would result in increased performance of the heat exchanger and less frequent maintenance issues for the heat exchanger.